1. Field of the Invention
The present invention relates to a display device and a driving method of a display device, and particularly to an active matrix type display device formed by two-dimensionally arranging pixels each including an electrooptic element in the form of a matrix and a driving method of the display device.
2. Description of the Related Art
A display device formed by arranging pixels each including an electrooptic element in the form of a matrix, for example an active matrix type liquid crystal display device formed by two-dimensionally arranging pixels each including a liquid crystal cell, the liquid crystal cell being used as an electrooptic element, in the form of a matrix generally employs an alternating-current driving system that inverts the polarity of a display signal in certain periods with a common potential Vcom as a center in order to prevent degradation of liquid crystal and image burn-in in an alignment layer due to continuous application of a direct-current voltage of the same polarity to the liquid crystal.
FIGS. 13A and 13B are diagrams of assistance in explaining a field inversion driving system that inverts the polarity of a display signal in field periods. FIG. 13A shows, for example, a pixel arrangement of four rows and four columns. FIG. 13B shows a driving waveform for each pixel in the pixel arrangement.
This field inversion driving system has a problem of degradation in display quality due to a so-called vertical crosstalk caused by a leak of pixel transistors for switching liquid crystal cells. Specifically, as shown in FIG. 14, when a normally white type liquid crystal display device (a liquid crystal display device that decreases transmittance as voltage applied to liquid crystal is raised), for example, displays a black window on a gray background, a problem occurs in pixels in gray areas 02 and 03 situated in a direction of vertical scanning (top-to-bottom direction) of a black area 01 in that the pixels in the area 02 over the black area 01 appear darker than original gray and the pixels in the area 03 under the black area 01 appear lighter than the original gray.
The problem of this vertical crosstalk occurs because field inversion driving switches between positive polarity driving and negative polarity driving in field units, and thereby changes potentials between the common electrodes of pixels, source wiring, and gates, resulting in a difference between an amount of leakage (amount of crosstalk) of pixel transistors in the upper area 02 and an amount of leakage (amount of crosstalk) of pixel transistors in the lower area 03.
Making more concrete description, when the pixels are written with a positive polarity (or a negative polarity) in a certain field, and written with a negative polarity (or a positive polarity) in a next field, in a stage of writing the black area 01, the polarity of a row being written and the polarity of the upper area 02 are the same negative polarity, whereas the polarity of the lower area 03 yet to be written remains the positive polarity of the previous field.
Thus, with respect to a potential to be written to the black area 01, the polarity of a potential retained by the pixels in the upper area 02 is different from the polarity of a potential retained by the pixels in the lower area 03, resulting in a difference between the amounts of leakage of the pixel transistors in the upper area 02 and the lower area 03. Therefore, the upper area 02 over the black area 01 appears darker than the original gray, and the lower area 03 under the black area 01 appears lighter than the original gray.
To deal with degradation in display quality due to such a vertical crosstalk in related art, image data for each pixel is corrected such that even when the potential of a pixel electrode is changed with a potential change, the potential of the pixel electrode coincides with an average potential within a frame when it is assumed that the change in the potential of the pixel electrode does not occur (see for example, Japanese Patent Laid-Open No. 2005-077508. Hereinafter refer to as Patent Document 1.).
Also known is a technique that uses a memory (line memory) having a capacity for one scanning line, stores a sum of information for one vertical column in a previous field, and corrects image data for each pixel in a present field using the information stored in the line memory (see for example, Japanese Patent Laid-Open No. 2000-330093. Hereinafter refer to as Patent Document 2).